what size throttle body for adequate air flow?

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Re: what size throttle body for adequate air flow?

Postby Daryl » Sat Jan 07, 2017 10:35 pm

Why not just match the diameter/area of the intake ports?
Intake ports on my engine are 36mm diameter, I don't see the point in putting 40 or 42 mm itb's, where the flow will be choked off by the port....??
If I put on 36mm itb's, maybe 37 or 38's to allow for restriction of butterfly and throttle shaft (and carefully taper where the two meet), won't that allow for best flow velocity, regardless of engine displacement?
What happens to flow if you use itb's that are smaller than the ports?
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Re: what size throttle body for adequate air flow?

Postby whittlebeast » Sun Jan 08, 2017 5:26 am

I have found that with ITBs, it is tough to find air filters that allow enough air flow (no MAP drop at WOT) that will fit in the chassis.

http://www.nbs-stl.com/CrxTypeR/B18C%20Side.jpg

http://www.nbs-stl.com/CRX/Dyno%20Pull%20Graph.png

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Re: what size throttle body for adequate air flow?

Postby Paul_VR6 » Sun Jan 08, 2017 8:11 am

In general you want at least 2mm bigger than the port diameter and usually 5mm is what I would use. The further out from the head they are the bigger you want them (the port should taper).

I agree on the filters, thats why those plenum/airboxes exist.
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Re: what size throttle body for adequate air flow?

Postby whittlebeast » Sun Jan 08, 2017 8:18 am

The problem with plenum air boxes is keeping the ports from effecting each other. The new motor with the throttle response may prove to be a bit of a challange to drive in the pits. We will see.

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Re: what size throttle body for adequate air flow?

Postby Paul_VR6 » Sun Jan 08, 2017 8:56 am

Even in open air they effect each other slightly. The bigger the box the less they have an effect on each other.

We did a little experiment with a big hp four cyl and made 5whp across the board by tilting the horns up/down staggered instead of siamesing them. It was wild.
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